This invention relates to conjugates of antibodies and therapeutic agents and, more specifically, to therapeutic preparations of actinomycin D linked to an antibody for delivery of actinomycin D to the site of action as dictated by the antibody specificity.
It has long been recognized as highly desirable to specifically target therapeutic agents to invading organisms or diseased cells. Specific targeting permits lower doses of the therapeutic agent to be given and reduces the observed side effects from non-specific action of the agent. Various target cell binding agents, such as liposomes, proteins and antibodies, have been used in conjunction with pharmaceutical or cytotoxic agents, such as microbial toxins, protein synthesis inhibitors (i.e. diphtheria toxin) and radioactive compounds as specific targeting molecules.
Antibodies, particularly monoclonal antibodies, which recognize specifically selected antigens are especially suited for delivery of therapeutic agents. Monoclonal antibodies are advantageous since they have the ability to recognize a single molecular site or epitope on a cell. Studies have identified monoclonal antibodies specifically directed toward tumor-associated antigens and other antigens on cancer cells, T-cells and B-cells. These monoclonal antibodies can be used to deliver drugs directly and specifically to the target cells. One of the potential means of attacking such target cells is to use the monoclonal antibodies with an attached drug, such as an antibiotic. Delivery of antibody-therapeutic agent to specific cells, tissues, organs, or any other site in vivo can be accomplished using whole antibodies or fragments of antibodies. Fragments, such as Fab, can be used in place of whole antibodies if they retain the ability to recognize selected antigens.
Conjugates of antibody-therapeutic agent can be made by chemically coupling the two through covalent bonds. One disadvantage of covalent attachment to the backbone of an antibody molecule is that if the chemical modification is in the antigen binding region, the recognition of the antibody can be changed. This adverse effect on the functional properties of the antibody has been a problem with the random linkage of drugs to antibodies. The critical features of the resulting conjugate are that it maintain its biological activity, (both the antibody and therapeutic agent) and that it be stable for use in vivo. The attachment of the therapeutic agent to the target cell binding protein must be stable in all conditions of administration to a patient and under all conditions present in the microenvironment at the site of action. Further, for administration of an effective amount in a human or animal, the conjugate must remain immunospecific for an antigenic determinant on specific cells or tissues.
Actinomycin D is an antibiotic which inhibits RNA transcription, the process by which genetic information in one strand of DNA is copied into a complementary set of bases called messenger RNA (mRNA). Actinomycin D intercalates, or inserts itself, into the double-helical DNA between G.tbd.C base pairs causing a deformed DNA template, thereby preventing efficient RNA transcription. In essence, actinomycin D jams the DNA zipper.
Further, actinomycin D is especially attractive due to its effectiveness at low concentrations. In low concentrations, actinomycin D inhibits transcription without appreciably affecting DNA replication or protein synthesis. It has been extensively used as a highly specific inhibitor of the formation of new RNA in both procaryotic and eucaryotic cells which eventually results in the death of the cells. Additionally, its inhibition of the growth of rapidly dividing cells makes it an effective non-selective therapeutic agent in the treatment of some cancers.
The chemistry for attaching actinomycin D to selective targeting agents has been burdened with unwanted side products and all attempts have been unsuccessful.
There thus exists a need for target cell binding agents conjugated to actinomycin D which can be used for therapeutic or diagnostic purposes. Preferably, such a conjugate should be stable in vivo. The present invention satisfies these needs and provides related advantages as well.